1. Field of the Invention
The present invention relates to an image reading apparatus which is capable of reading both sides of a document, and an image forming apparatus provided with the image reading apparatus.
2. Description of the Related Art
Conventionally, in image forming apparatuses such as a copying machine and a facsimile machine, front and back sides of a document is read automatically by an image reading apparatus provided with an automatic document feeding device (ADF: Automatic Document Feeder). In this document reading, a method of reading an image on a front side of a document with an image sensor, reversing the document front and back, and reading an image on a back side of the document with the image sensor again has been widely adopted.
Further, as shown in a patent document 1 (Japanese Patent Unexamined Publication No. 2007-82033), there has been a known image forming apparatus adopting a method of providing image sensors respectively for both front and back sides of a document in a document conveying passage from a document tray to substantially simultaneously reading both sides of the document at one document conveyance (one-path) without reversing the document. In the image forming apparatus adopting the method of substantially simultaneously reading both sides of the document, it is likely that reading a one side of the document is performed with a minification optical system image sensor provided on a side of an apparatus main body, and reading the other side of the document is performed with a contact type optical system image sensor provided on a side of the automatic document feeding device in view of installation space, cost, and the like.
If optical configurations of image sensors for reading front and back sides of a document are different from one another like the image forming apparatus shown in the patent document 1, a difference in image quality occurs in a read image due to difference in light sources and lens arrays used respectively. Further, the difference in image quality of the read images appear as a difference in image quality of images formed on the front and back sides of the recording sheet. Therefore, in such image forming apparatus, for the purpose of obtaining the same image quality on the front and back sides of the recording sheet, there are provided a double-sided reverse reading mode of reversing one sheet of document, conveying the document twice to the same image sensor (minification optical system image sensor), and allowing the image sensor to read front and back sides of the document, and a double-sided simultaneous reading mode of using two image sensors of a minification optical system and a contact type optical system to read front and back sides of the document substantially simultaneously to shorten an image reading time, so that a user can select a desired mode.
However, in the image forming apparatus shown in the patent document 1, as shown in its FIG. 12, the contact optical system image sensor for reading the other side of a document is provided on a document conveying passage at a position on a downstream side in a document conveying direction from the minification optical system image sensor for reading one side of the document and on an upstream side in the document conveying direction from the document reversing mechanism. In such image forming apparatus, the contact optical system image sensor is disposed in a loop of a circular document conveying passage (which is formed by a first document conveying passage for connecting the document tray and the minification optical system image sensor, a reverse document conveying passage connecting the minification optical system image sensor and the document reversing mechanism, and a re-conveying document conveying passage connecting the document reversing mechanism and the first document conveying passage) for putting a document back to the minification optical system image sensor again in the double-sided reverse reading mode. Therefore, the loop becomes large by a space for providing the contact optical system image sensor. This causes the conveying passage for putting the document back to the minification optical system image sensor again in the double-sided reverse reading mode to be long, so that a long time is required for reading a document in the double-sided reverse reading mode.